The present invention relates to a built-up girder structure.
The present invention also relates to a method for producing such a girder structure.
Many varieties of cross sections of girders intended for various uses are obviously known. The most effective cross sections are those with a maximum metal content in the regions that are most remote from the neutral fibre.
More particularly, the standard I-girder, which comprises two flanges linked by a web, is a good solution, since the material located at the level of the flanges will determine the moment of inertia. In an elastic regime, the stresses and deformations linearly vary in the cross section: they are equal to zero at the neutral fibre and increase until they are maximal at the point most remote from the neutral fibre. The web, which securely fastens the two flanges together, is thus the site of bending stresses associated with the local moment, shear stresses associated with the local transverse force, and compressive stresses determined by the local loading.
It has recently been sought to produce lighter girder structures by reducing the amount of working material. In particular, it has been suggested to use shades of carbon steel with high mechanical strength characteristics, often associated with formability by limited deformation. The cost prices of the steels thus produced by known bulk metallurgical methods are similar to the prices of standard carbon steels and allow the production of lighter structures, being thus economically more advantageous.
To clarify matters, we will differentiate these steels according to their elastic limit (EL) in the remainder of the description:
mild steels: EL less than 250 MPa;
steels with a high elastic limit (HEL):
250 MPa less than EL less than 600 MPa;
steels with a very high elastic limit (VHEL):
600 MPa less than EL less than 1000 MPa;
steels with an ultra-high elastic limit (UHEL):
1000 MPa less than EL less than 1500 MPa.
The steels with high mechanical characteristics mentioned in the present patent application mainly belong to the VHEL and especially the UHEL category.
Nevertheless, on account of their low formability and their sometimes poor weldability, these steels pose certain specific problems during assembly for example. In particular, the standard methods for producing or preparing girder structures are generally only suitable for producing a girder of permanent cross section, which obviously does not allow to optimise the weight of said girder structure.
The concept of a built-up girder is known. Thus, document DE-A-22 21 330 proposes a built-up bending girder, the flanges and web of which respectively consist of very high strength steel and of ordinary steel. The apparent elastic limit is exceeded in the region of the web close to the flange, but it is precisely the junction with the very high strength steel, maintaining an elastic behaviour, which prevents the web from flowing. A girder entirely consisting of very high strength steel and having the same behaviour as a girder of the same dimensions is thus obtained.
Similarly, document FR-A-1 312 864 describes an I-girder consisting of three welded parts and especially having a first flange made of low-carbon steel and a second flange made of high-carbon steel. The latter flange is intended to be used as a rail.
Document GB-A-2 187 409 proposes to reinforce the flanges of a steel girder by bonding additional strips made of steel with a different shade, of another metal or alternatively even of plastic.
Document U.S. Pat. No. 3,999,354 describes an aluminium girder with a rectangular hollow cross section, consisting of two extruded and profiled flanges assembled with two webs in the form of a panel. Assembly is obtained at each junction by local pinching: an arm belonging to the flange is folded by means of a tool into a groove of the same flange, the corresponding web panel being wedged between these two elements. This type of mechanical assembly is relatively incompatible with VHEL and UHEL steels, which have poor formability qualities.
Other techniques for assembling girders are known and described in documents U.S. Pat. No. 3,960,637 and U.S. Pat. No. 5,483,782 for example.
Patent application DE-A-34 25 495 describes an I-girder with a web reinforced by regular mouldings. This reinforcement is necessary when girders with webs of a certain height are used.
Document FR-A-1 234 371 proposes methods and devices for implementing welded alveolar girders.
The present invention aims to propose a girder structure allowing to reduce the weight thereof, while at the same time to use steel plates with high mechanical characteristics.
The present invention also aims to allow the efficient production of girder structures with variable cross section.
The present invention also relates to the method for producing a girder structure such as described above in a particularly efficient manner.
The present invention relates to a girder structure comprising at least one flange (1, 1xe2x80x2) made of at least one first metal with high mechanical characteristics, i.e. a high resistance, having an elastic limit/breaking load ratio close to 1, and at least one web (2, 2xe2x80x2) made of at least one second metal having an elastic limit substantially inferior to that of the first metal, said web being essentially assembled perpendicular to said flange, said flange and said web being made of sheet or plate metal, characterized in that:
the second metal has an elastic limit/breaking load ratio substantially inferior to the value of said ratio of the first metal and of less than 0.9.
said web having geometrical characteristics that increase its buckling strength in comparison with a flat and full web of the same thickness and the same height, and decrease its thickness and consequently lower the total weight of the structure.
Preferably, the first metal is a steel with an elastic limit higher than 400 MPa or an aluminium alloy with an elastic limit higher than 200 MPa.
The webs may have corrugation, and in particular a succession of lances or apertures in the longitudinal direction of the girder structure.
Preferably, the girder structure comprises at least two flanges, at least one of which is made of the first metal, which are essentially parallel to each other and essentially assembled perpendicular to at least one element made of the second metal in order to produce a web.
According to the invention, the two flanges are made of the same metal, optionally of different thicknesses, or of different metals, a first flange being made of a metal with an elastic limit/breaking load ratio different from that of the metal of the other flange.
According to one particularly advantageous embodiment of the invention, the girder structure comprises at least two flanges essentially parallel to each other and connected together by at least two webs that are also essentially parallel to each other, in which the flanges and the webs are made of metallic materials that differ in their nature, their mechanical properties or their thickness.
In a particularly advantageous manner, the girder structure has a non-permanent cross section, which varies according to the height and/or width of said structure.
The invention also relates to a method for assembling a girder structure, comprising at least a flange and at least a web, such as those mentioned above, characterized in that said flange and said web are assembled in order to form a junction section by means of a fusion assembly method, preferably by spot welding, laser welding, seam welding, diffusion welding or brazing.
Alternatively, the assembly of said flange and said web in order to form a junction section is performed by a mechanical assembly method, preferably by riveting, simple crimping or clinching.
In a particularly advantageous manner, the assembly of said flange and said web in order to form a junction section is performed by an assembly method by hem crimping.
In this specific case, the ratio of the hem radius to the sum of the thicknesses of the various constituent elements along the junction section is preferably between 2 and 10. Similarly, the ratio of the difference between the radius of the hem and the thickness of the outermost constituent element to the thickness of the innermost constituent element is preferably higher than 2, and the thickness ratio of the two elements is preferably lower than 4.
The mechanical assembly operations (riveting, crimping, clinching or hem crimping) are preferably performed by means of a press.
Advantageously, the assembly by hem crimping is performed in the same press cycle.
In a particularly advantageous manner, the blocking of the hem made by the assembly method according to the invention with respect to the relative sliding of a web relative to a flange along the junction section may be achieved by bonding, indentation or imbrication.